It is important to the function of an elevator that its load be known at all times--whether to determine if the car is overloaded so that the elevator brake is not released while under that load or to determine which floor should be serviced based on the load in the car. Several types of elevator load weighing systems have been used.
Elevator load weighing systems may be divided into two groups--those that place a sensor beneath the cab platform and those that do not.
Several systems have been designed with the load weigher beneath the car. Some measure the deformation of resilient pads beneath the car platform, U.S. Pat. No. 4,078,623. A disadvantage of the deformation systems is the inaccuracy introduced by hysteresis in the elastomeric material used in the pads. U.S. Pat. No. 4,899,852 discloses an elevator car disposed in a frame which moves on rails through a hoistway; a pendulum mount is used to mount the car in the frame so that the car is free to swing from four suspension rods within the frame in pendulum fashion. Two of the four suspension rods extend through a support pad, a rubber pad, and a second support pad. There is no positive connection between the load cell and support pads. The two support pads sandwich the rubber pad. Below two corners of the car, a load cell separates the pads. This device involves deflection in the load cell in response to weight on the cab floor. Devices, like that of U.S. Pat. No. 4,899,852, involve deflection in response to the weight exerted on the cab floor. There are several problems. First, there is drift in the signal produced by the actuator due to hardening of the elastomeric pads. Typically, the platform is supported at some places by wooden blocks and rubber pads and at other places by load cells supported by rubber pads. But the hardness of the rubber pads increases from the time the elevator is installed. And, the hardness of different rubber pads increases at different rates, such that a load evenly distributed on a cab platform is not carried equally by different pads. In addition, accurate load measurement requires that the load cells and rubber pads should be under uniform pressure; achieving this is time consuming and difficult. Further, the load weight measurement typically depends on a linear relationship, over a range of weights to be measured, between deflection of a load cell actuator and a load of, for example, 0 to 3000 pounds. On installation, the relationship may be linear but because of the aging of the rubber pads, it becomes nonlinear for a portion of the range of weights to be measured. The result is an erroneous load measurement. When error in the load measurement becomes excessive, the use of the load weigher is undermined almost totally. For example, if the average elevator passenger weighs 150 pounds and the range is 0 to 2000 pounds, the resolution error is greater than 5% so that it cannot be determined whether or not one person is in the car. Frequently, adolescents will get on an elevator and press all the buttons. Anti-nuisance software estimates the number of people in a car by assuming an average weight per person. It then determines the number of car calls and if the number of car calls exceeds the number of people estimated call, it cancels those car calls. If a load weigher cannot distinguish the weight of an average person, this software cannot operate.
It is desirable to weigh the load in a manner independent of the distribution of the load within the cab. Co-pending application "Elevator Load Weighing at Car Hitch" discloses achieving this by means of a load cell and support sandwiched between a top and bottom hitch plate, the load being shared between the load cell and support. Threaded rods pass through the top and bottom plates, as well as the space between the top and bottom hitch plates, and further pass through springs and connect to spring seats and nuts on rods. The upper ends of the rods are connected to the hoist ropes. The load weighing apparatus further includes a pair of rod stabilizers which keep constant the lateral displacement of the rods one from another. The load cell and support are not enclosed by the rods, yielding the further advantage that a mechanic has easy access to the load cell for its maintenance. However, this solution has two problems. The first problem is the cause of slippage in the ropes and, because the relative position of the rope and sheave groove is not constant, different load measurements are obtained depending upon whether the elevator is traveling up or down when it arrives at a given position, and whether it has arrived at a given position twice. In other words, when the actual tension in the individual rope supporting the bottom hitch plate changes due to individual sheave groove diameter differences, the actual center of load in the bottom hitch plate will wander. This wandering center of load has the potential of introducing significant error in the load measurement by means of altering the effective ratio of loading between the load cell and other support points. For example, the load measurement at floor 5 when traveling in the up direction is different from the load measurement obtained when traveling to floor 5 in a down direction. In addition, the load measurement at floor 5 after traveling from floor 6 to floor 5 is different from the load measurement obtained after traveling from floor 7 to floor 5.
The second problem with the above hitch load weighing device is that there is no overload protection on the load cell.